Screening process simulation apparatus

ABSTRACT

Disclosed is apparatus useful for electronically simulating the variable graphic arts process step of photographically screening a continuous tone image to develop a corresponding halftone image. The apparatus is readily adjustable for operation over all or any selected portion of a basic nonlinear signal translation characteristic, and thus is capable of simulating any of a plurality of different screening processes and variations in the adjustable parameters of any single screening process. Other embodiments are covered.

United States Patent Inventor Rudolph Moraenfruh 3,l28,338 4/1964 Bailey l78/5.2 [21] APP] No 332:3 3,131,252 4/1964 Farberetal. 178/52 [22] Filed. 6, 1969 Primary ExaminerRobert L. Griffin [45] patented Deg 21, Assistant Examiner-Barry Leibowitz [73] Assign m Corporation Attorney-Kenneth P. Robinson [54] SCREENING pROCESS SIMULATION APPARATUS ABSTRACT: Disclosed is apparatus useful for electronically 10 ChimsADl-awing Figs simulating the variable graphic arts process step of photo- U S Cl graphically screening a continuous tone image to develop a 178/6, corresponding halt-tone image The apparatus is read"), Int Cl l78/6'8 justable for operation over all or any selected portion of a [so] H041! 3/30 basic nonlinear signal translation characteristic and thus i capable of simulating any of a plurality of different screening A processes and variations in the adjustable parameters of any [56] References Cited single screening process. Other embodiments are covered.

UNITED STATES PATENTS 3,123,666 3/1964 Teacheretal. n 178/68 A i ""1.- T2 ".3 'Ti -i 0.0. LEVEL SIGNAL 1 INPUT-a I AMP.

! CIRCUIT a fillies? OUTPUT 1 1 i g 65 gun; I I g l I l l J PATENTEU UECZI lsn SHEET 2 [IF 2 PDOA m I I 2 I I l I I: I I I SCREENING PROCESS SIMULATION APPARATUS The present invention relates generally to systems for electronically simulating graphic arts processes, and more particularly to apparatus for use in such systems to electronically simulate the variable process step of screening a continuous tone image to develop a halftone image.

The graphic arts industry is burdened with major inefficiencies attributable to the time consuming and otherwise costly trial and error aspects of the various printing processes used. The necessity for such trial anderror operation arises from the inherent nature of graphic arts processes, in that the product of any such process cannot be conveniently viewed and readily evaluated at any of the intermediate stages of the process. Only by viewing the final product (the resulting print) can it be determined whether or not the variable parameters of the printing process have been suitably adjusted, or whether or not the original input copy to such a process (i.e., color transparency, set of color separations, etc.) requires precorrection in order to produce characteristics in the final print which cannot be introduced via adjustment of the processes variable parameters.

It has been proposed that electronic equipment be constructed to function as the electronic analog of a complete graphic arts process, or at least that portion of the process which involves a significant amount of trial and error. An example of the latter is that portion which involves proofing a set of color separations by creating a set of printing plates therefrom, placing them on a printing press and running off an actual sample color print, or proof for evaluation. Equipment of this type is to develop an electronically displayed image which simulates the print that could be obtained by means of the actual graphic arts process, so that an operator can preview the efi'ects of all or a selected portion of the process before committing actual manpower, material and press equipment to the printing run. In view of the many different processes used in the graphic arts industry, it is desirable to have preview equipment of this type be as flexible as possible, and capable of accurately simulating a wide range of graphic arts processes and characteristics. To this end it would be particularly advantageous to have apparatus which could be used in such equipment, and which is readily adjustable so as to be capable of electronically simulating any of a plurality of different graphic arts screening processes and variations in the adjustable parameters of any single screening process.

It is therefore an object of the present invention to provide new and improved adjustable apparatus for electronically simulating the screening process step in graphic arts processes.

It is also an object of the invention to provide apparatus which is relatively simple in nature and yet adjustable to provide a wide range of signal translation characteristics so as to be capable of simulating any one of a number of different screening process steps or variations of the same screening process step in a graphic arts process.

In accordance with the present invention apparatus usable for simulating the graphic'arts process step of screening a continuous tone image to develop a corresponding halftone image comprises signal translation means, havinga predetermined nonlinear input signal amplitude to output signal amplitude translation characteristic, different portions of which are representative of different screening processes which can be simulated, for translating a supplied signal over a portion of the characteristic that corresponds to the amplitude range of the supplied signal. The apparatus further comprises input signal modifying means, responsive to an input signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying the input signal to develop a modified signal having amplitude variations proportional to those of theinput signal but extending over a selected second range of values corresponding to the portion of the translation characteristic of the signal translation means that represents a specific screening process being simulated, and for supplying the modified signal to the input of the signal translation means and translated signal modifying means, coupled to the output of the signal translation means, for modifying the translated signal to develop a resultant signal having amplitude variations proportional to those of the translated signal and extending over a selected third range of values and representative of a desired halftone image which can be developed from the continuous tone image by means of the specific screening process.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an electronic graphic arts process simulation system;

FIG. 2 is a block and schematic diagram of screening simulation apparatus constructed in accordance with one embodiment of the present invention and useful in the system of FIG.

FIG. 3 is a graph illustrating one suitable signal translation characteristic for the signal translation network of FIG. I,

and FIG. 4 is a schematic diagram illustrating a specific embodiment of the screening simulator 10 shown in FIG. 2.

DESCRIPTION AND OPERATION OF THE APPARATUS v The adjustable screening process simulating apparatus 10 of FIG. 2 is useful in a graphic arts process simulating system such as that of FIG. I to convert an input video signal representative of a continuous tone image, into an output video signal, representative of the corresponding halftone image that would be obtained by means of a selected photographic type screening process. To accomplish this in accordance with one embodiment of the present invention, there is supplied to the input of apparatus 10 of FIG. 2, a video signal whose instantaneous amplitude is representative of the density of successively scanned picture elements of a continuous tone photographic transparency, and whose peak-to-peak amplitude lies within a first range of voltage values. Techniques for developing such a video signal are well known in the art and may consist of scanning a continuous tone transparency with a TV-type raster via a beam of light from a flying spot scanner. The light transmitted by the transparency is then collected and detected to develop a video signal whose instantaneous amplitude is proportional to the transmissivity of the transparency. This initial video signal is then nonlinearly amplified to develop the required input video signal whose instantaneous amplitude is proportional to the density of successively scanned picture elements of the continuous tone transparency. This arrangement is shown in general block form in FIG. I by the blocks which precede the apparatus 10.

The density representative video signal resulting from the above is supplied to an amplifier 11, having an adjustable gain control shown schematically as the potentiometer 11a. The amplified video signal is then coupled to a direct current (DC) level set circuit 12, wherein the DC component of the amplified video signal can be increased or decreased by means of an adjustable level control shown schematically as the potentiometer 12a. This series combination of amplifier 11 and DC level set circuit 12, with their respective controls 11a and 12a, in the present embodiment constitutes means for adjustably modifying the input video signal to develop a modified video signal having amplitude variations proportional to those of the input video signal but extending over a selected second range of values corresponding to the portion of the translation characteristic of a signal translation means (described hereinafter) that represents the specific screening process to be simulated, and for supplying the modified video signal to the input of the signal translation means. The second range over which the amplitude variations of the modified video signal extend is selected by adjusting either or both of the adjustable controls Ila and 12a. By adjustment of the gain control 11a, the span between the upper and lower limits of the second range of voltages can be made to be different than that of the aforementioned first range. Correspondingly, by adjustment of the DC level control 12a, the entire second range of voltages can be raised or lowered in relation to the first range.

Amplifier II may be any suitable adjustable gain amplifier which is substantially linear over its range of gain settings. Similarly, DC level set circuit 12 may be any suitable circuit which permits adjustment of the DC component of a signal. An example of one such level set circuit, which is useful in the case where AC coupling is employed between different portions of the apparatus, is the gated DC clamp circuit 12' shown in the specific embodiment of FIG. 4. Circuit 12 clamps the blanking portion of the AC coupled video signal from amplifier 11 to a DC level selected by adjustment of the variable resistor 12a. This type of clamp circuit is well known, and thus will not be further described.

The modified video signal appearing at the output of circuit 12 in FIG. 2, then is coupled to the input of signal translation circuit 13, which constitutes means, having a predetermined nonlinear input signal amplitude to output signal amplitude translation characteristic, different portions of which are representative of different screening processes to be simulated, for translating the modified video signal supplied from circuit l2 over that portion of the characteristic which corresponds to the amplitude range (the aforementioned selected second range) of the supplied signal. The total overall signal translation characteristic of network 13 is fixed by design, and preferably is of an S-shape such as that shown in FIG. 3. The reasons for using an S-shaped characteristic will become clear hereinafter, but basically this characteristic gives apparatus constructed in accordance with the present invention a unique versatility that enables it not only to simulate many different screening processes, but also to simulate the variables which may be controlled in any one particular screening process, such as by varying the screen size, screen material, film type, exposure parameters, etc. It will be appreciated that in using apparatus which embodies the present invention, any specific screening process can be simulated by merely adjusting one or more of the three adjustable controls Ila, 12a, and 1411. It should be understood that the term screening process" as used herein, means the entire screening process or any portion thereof, since in some instances it may be desirable to simulate only a selected portion of a particular screening process with apparatus embodying the present invention. For example, it may be desirable in some instances to simulate a particular screening process by using the present invention to simulate the relatively fixed parameters of that process (i.e., those relating to size and type of screen, film type, etc.) and using other electronic analog circuitry to simulate those parameters of the process which are more readily variable (i.e., the exposure parameters of main, flash and bump).

Circuit I3 may be of any suitable design capable of presenting an S-shaped signal translation characteristic, such as that of FIG. 3. A conventional design technique suitable for use in constructing circuit 13 is that of piecewise linear approximation, using a resistor-diode network such as that shown in the specific embodiment of FIG. 4. In the circuit 13' of FIG. 4, diodes 15, I6 and 17 sequentially switch out of the circuit at progressively higher input signal amplitudes to develop the lower concave portion (a) of the signal translation characteristic of FIG. 3, whereas diodes 13, I9 and 20 sequentially switch into the circuit at progressively even higher input signal amplitudes to develop the upper convex portion (b) of the FIG. 3 characteristic. The transistor pair 0,, function as an isolation amplifier presenting a high input impedance to the preceding unit 12 and a low driving impedance to the resistor-diode network.

The translated video signal appearing at the output of network 13 in FIG. 2, is then coupled to the input of an amplifier 14, which is equipped with an adjustable gain control shown schematically as the potentiometer 14a. Amplifier 14 constitutes means, coupled to the output of the signal translation means 13, for modifying the translated signal to develop a resultant signal having amplitude variations proportional to those of the translated signal but extending over a selected third range of values and representative of the desired halftone image to be developed by the specific screening process being simulated from the continuous tone image represented by the original video signal supplied to the input of apparatus 10. Amplifier 14 may be any conventional adjustable gain amplifier which is substantially linear over its range of gain control settings.

From the foregoing, the operation of the individual units 1 I l2, l3 and 14 should be clear and will not be described further. However, the novel manner in which these units cooperatively function to simulate virtually any desired screening process will now be discussed.

The various screening processes used in the graphic arts industry can be electronically simulated in accordance with the invention by apparatus, such as that of FIG. 2, which is capable of exhibiting three types of input-to-output signal translation characteristics, namely (I) concave upward, (II) concave downward, and (III) compound concave upward-downward. The apparatus of FIG. 2 achieves this capability in a particularly simple manner in accordance with the present invention by utilizing a single signal translation circuit I3 which is designed to have a fixed S-shaped characteristic such as that shown in FIG. 3. From the above it will be appreciated that different portions of the total S-shaped signal translation characteristic of circuit 13 are representative of different screening processes which can be simulated. Thus, by merely adjusting one or both of the controls 11a and IZa, the apparatus of FIG. 2 can be made to operate over any selected portion of the S-shaped characteristic of circuit 13. By operating only over the lower half of the characteristic (portion "a" in FIG. 3), the apparatus can be made to exhibit an effective input-to-output signal translation characteristic of the form (I) noted above, namely, concave upward. Similarly, by operating only over the upper half of the S-shaped characteristic (portion b" in FIG. 2), the apparatus will exhibit an effective input-to-output characteristic which is of the form (ll) above, namely, concave downward. Finally, by operating over the whole S-shaped characteristic of circuit 13, the apparatus will exhibit an input-to-output characteristic that is of the form (III) above, namely, compound concave upward-downward. In addition, by operating only over the central portion of the S-shaped characteristic adjacent to the point of inflection, the apparatus can also be made to exhibit an effective input-tooutput signal translation characteristic which is substantially linear in form.

Selection of that portion of the S-shaped characteristic over which it is desired to operate the apparatus is accomplished by adjustably modifying the amplitude and DC component of the video signal supplied to the input of apparatus 10, in the amplifier 11 and DC level set circuit 12, respectively. In addition, the slope of the selected portion of the S-shaped characteristic can be effectively altered by adjusting the gain of the amplifier 14, thereby introducing a linear change in the amplitude of the translated signal developed at the output of the signal translation circuit 13. Thus, by simply adjusting one or more of the controls 11a, 12a and 14a, the apparatus of FIG. 2 can be made to exhibit the input-to-output signal translation characteristic necessary to simulate any graphic arts screening process.

As an example, the screening simulator I0 of FIG. 2 may be used in an electronic graphic arts process previewer such as disclosed in copending application Ser. No. 874,550, filed Nov. 6, 1969, and assigned to the same assignee as the present case. In such a case the controls of the screening simulator 10 are manipulated until an acceptable color preview image appears on the previewers image display screen. The screening simulator's input is then disconnected and a recurring ramp signal supplied thereto, while observing its output on an oscilloscope. The oscilloscope displays the signal translation characteristic exhibited by the screening simulator with its controls set as desired. The observed translation characteristic can then be compared with the characteristics of various actual screening processes, which have previously been measured and catalogued for use with the invention, until an approximate match is found. Together with the measured characteristics of the matching actual screening process, the catalog indicates what values the various parameters of that process must have in order to duplicate the desired screening characteristic (i.e., screen size, film type, main-flash-bump exposure values, etc.). With these values known, the actual screening process can be preset so as to create the desired halftone transparency from the original continuous tone transparency in a single operation.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention,

What is claimed is:

1. Apparatus for independently and electronically simulating the graphic arts process step of screening a continuous tone image to develop a corresponding halftone image, comprising:

signal translation means, consisting of a single signal translating channel having a predetermined nonlinear input signal amplitude to output signal amplitude translation characteristic different portions of which are proportional to the continuous-tone to halftone conversion that occurs in corresponding different screening processes to be simulated, for translating a supplied signal over a portion of said characteristic determined by the range of amplitude variations of the supplied signal;

adjustable input signal modifying means, responsive to an input signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying said input signal to develop a modified signal having amplitude variations proportional to those of said input signal but extending over a selected second range of values corresponding to the portion of said nonlinear signal translation characteristic that represents a selected one of said different screening processes and for supplying said modified signal to the signal translating channel of said signal translation means;

and adjustable translated signal modifying means, coupled to the output of the signal translating channel of said signal translation means, for modifying said translated signal to develop a resultant signal having amplitude variations proportional to those of said translated signal and extending over a selected third range of values and representative of a desired halftone image which can be developed from said continuous tone image by means of said selected screening process.

2. Apparatus constructed in accordance with claim 1 wherein the signal translating channel of said signal translation means has an input-to-output signal translation characteristic that is fixed and substantially S-shaped in form.

3. Apparatus in accordance with claim 2 wherein said means for modifying the input signal comprises means for modifying the peak-to-peak amplitude of said input signal thereby to develop said modified signal.

4. Apparatus constructed in accordance with claim 2 wherein said means for modifying the input signal comprises means for modifying both the peak-to-peak amplitude and the DC component of said input signal thereby to develop said modified signal.

5. Apparatus constructed in accordance with claim 2 wherein said means for modifying the translated signal comprises means for modifying the peak-to-peak amplitude of said translated signal to develop said output signal.

6, Apparatus for independently and electronically simulating the graphic arts process step of screening a continuous tone image to develop a corresponding halftone image, comprising:

signal translation means, consisting of a single signal translating channel having a predetermined nonlinear input signal amplitude-to-output signal amplitude translation characteristic different portions of which are proportional to the continuous-tone to halftone conversion that occurs in corresponding different screening processes to be simulated, for translating a supplied signal over a portion of said characteristic determined by the range of amplitude variations of the supplied signal;

first signal amplitude modifying means, responsive to an input signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying the amplitude of said input signal to develop a first modified signal having amplitude variations proportional to those of said input signal but extending over a selected different range of values;

means for modifying the direct current component of said first modified signal to develop a second modified signal having amplitude variations proportional to those of said first modified signal but extending over a selected range of values corresponding to the portion of said nonlinear signal translation characteristic that represents a selected one of said different screening processes and for supplying said second modified signal to the signal translating channel of said signal translation means;

and second signal amplitude modifying means, coupled to the output of the signal translating channel of said signal translation means, for adjustably modifying the amplitude of said translated signal to develop an output signal having amplitude variations proportional to those of said translated signal and extending over a selected third range of values and representative of the desired halftone image which can be developed from said continuous tone image by means of said selected screening process.

7. Apparatus constructed in accordance with claim 6 wherein the signal translating channel of said signal translation means has an input-to-output signal translation characteristic that is fixed and substantially S-shaped in form.

8. Apparatus constructed in accordance with claim 7 wherein said first and second signal amplitude modifying means each comprises an adjustable gain amplifier, for modifying the peak-to-peak amplitude of said input and translated signals, respectively, thereby to develop said first modified signal and said output signal, respectively.

9. Apparatus constructed in accordance with claim 7 wherein said means for modifying the DC component of said first modified signal comprises an adjustable DC clamp circuit for clamping a reference portion of the first modified signal to a selected DC reference level, thereby developing said second modified signal.

10. In an electronic system for simulating graphic arts processes, apparatus for independently simulating the process step of screening a continuous tone image to develop a corresponding halftone image, comprising:

a single nonlinear amplifier, consisting of a single signal translating channel having a predetermined nonlinear input signal amplitude to output signal amplitude translation characteristic that is fixed and substantially S-shaped in form and different portions of which are proportional to the continuous-tone to halftone conversion that occurs in corresponding different screening processes to be simulated, for translating a supplied video signal over a portion of said characteristic determined by the range of amplitude variations of the supplied signal;

a first adjustable gain amplifier, responsive to an input video signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying the peak-to-peak amplitude of said 7 input video signal to develop a first modified video signal having amplitude variations proportional to those of said input video signal but extending over a selected different range of values;

a direct current level set circuit for clamping a reference portion of said first modified video signal to an adjustable direct current reference level, thereby to develop a second modified video signal having amplitude variations proportional to those of said first modified signal but extending over a selected second range of values corresponding to the portion of said nonlinear signal translation characteristic that represents a selected one of said different screening processes and for supplying said second modified video signal to the input of said nonlinear amplifier;

and a second adjustable gain amplifier, coupled to the out- 

1. Apparatus for independently and electronically simulating the graphic arts process step of screening a continuous tone image to develop a corresponding halftone image, comprising: signal translation means, consisting of a single signal translating channel having a predetermined nonlinear input signal amplitude to output signal amplitude translation characteristic different portions of which are proportiOnal to the continuous-tone to halftone conversion that occurs in corresponding different screening processes to be simulated, for translating a supplied signal over a portion of said characteristic determined by the range of amplitude variations of the supplied signal; adjustable input signal modifying means, responsive to an input signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying said input signal to develop a modified signal having amplitude variations proportional to those of said input signal but extending over a selected second range of values corresponding to the portion of said nonlinear signal translation characteristic that represents a selected one of said different screening processes and for supplying said modified signal to the signal translating channel of said signal translation means; and adjustable translated signal modifying means, coupled to the output of the signal translating channel of said signal translation means, for modifying said translated signal to develop a resultant signal having amplitude variations proportional to those of said translated signal and extending over a selected third range of values and representative of a desired halftone image which can be developed from said continuous tone image by means of said selected screening process.
 2. Apparatus constructed in accordance with claim 1 wherein the signal translating channel of said signal translation means has an input-to-output signal translation characteristic that is fixed and substantially S-shaped in form.
 3. Apparatus in accordance with claim 2 wherein said means for modifying the input signal comprises means for modifying the peak-to-peak amplitude of said input signal thereby to develop said modified signal.
 4. Apparatus constructed in accordance with claim 2 wherein said means for modifying the input signal comprises means for modifying both the peak-to-peak amplitude and the DC component of said input signal thereby to develop said modified signal.
 5. Apparatus constructed in accordance with claim 2 wherein said means for modifying the translated signal comprises means for modifying the peak-to-peak amplitude of said translated signal to develop said output signal.
 6. Apparatus for independently and electronically simulating the graphic arts process step of screening a continuous tone image to develop a corresponding halftone image, comprising: signal translation means, consisting of a single signal translating channel having a predetermined nonlinear input signal amplitude-to-output signal amplitude translation characteristic different portions of which are proportional to the continuous-tone to halftone conversion that occurs in corresponding different screening processes to be simulated, for translating a supplied signal over a portion of said characteristic determined by the range of amplitude variations of the supplied signal; first signal amplitude modifying means, responsive to an input signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying the amplitude of said input signal to develop a first modified signal having amplitude variations proportional to those of said input signal but extending over a selected different range of values; means for modifying the direct current component of said first modified signal to develop a second modified signal having amplitude variations proportional to those of said first modified signal but extending over a selected range of values corresponding to the portion of said nonlinear signal translation characteristic that represents a selected one of said different screening processes and for supplying said second modified signal to the signal translating channel of said signal translation means; and second signal amplitude modifying means, coupled to the output of the signal translating channel of said signal translatiOn means, for adjustably modifying the amplitude of said translated signal to develop an output signal having amplitude variations proportional to those of said translated signal and extending over a selected third range of values and representative of the desired halftone image which can be developed from said continuous tone image by means of said selected screening process.
 7. Apparatus constructed in accordance with claim 6 wherein the signal translating channel of said signal translation means has an input-to-output signal translation characteristic that is fixed and substantially S-shaped in form.
 8. Apparatus constructed in accordance with claim 7 wherein said first and second signal amplitude modifying means each comprises an adjustable gain amplifier, for modifying the peak-to-peak amplitude of said input and translated signals, respectively, thereby to develop said first modified signal and said output signal, respectively.
 9. Apparatus constructed in accordance with claim 7 wherein said means for modifying the DC component of said first modified signal comprises an adjustable DC clamp circuit for clamping a reference portion of the first modified signal to a selected DC reference level, thereby developing said second modified signal.
 10. In an electronic system for simulating graphic arts processes, apparatus for independently simulating the process step of screening a continuous tone image to develop a corresponding halftone image, comprising: a single nonlinear amplifier, consisting of a single signal translating channel having a predetermined nonlinear input signal amplitude to output signal amplitude translation characteristic that is fixed and substantially S-shaped in form and different portions of which are proportional to the continuous-tone to halftone conversion that occurs in corresponding different screening processes to be simulated, for translating a supplied video signal over a portion of said characteristic determined by the range of amplitude variations of the supplied signal; a first adjustable gain amplifier, responsive to an input video signal having amplitude variations within a first range of values and representative of a continuous tone image, for adjustably modifying the peak-to-peak amplitude of said input video signal to develop a first modified video signal having amplitude variations proportional to those of said input video signal but extending over a selected different range of values; a direct current level set circuit for clamping a reference portion of said first modified video signal to an adjustable direct current reference level, thereby to develop a second modified video signal having amplitude variations proportional to those of said first modified signal but extending over a selected second range of values corresponding to the portion of said nonlinear signal translation characteristic that represents a selected one of said different screening processes and for supplying said second modified video signal to the input of said nonlinear amplifier; and a second adjustable gain amplifier, coupled to the output of said nonlinear amplifier, for adjustably modifying the peak-to-peak amplitude of said translated video signal to develop a resultant video signal having amplitude variations proportional to those of said translated video signal and extending over a selected third range of values and representative of the desired halftone image which can be developed from said continuous tone image by means of said selected screening process. 